1. Field of the Invention
The invention relates to papermakers felts and more particularly relates to a wet press felt for use in the press section of a papermaking machine and the method of its fabrication.
2. Brief Description of the Prior Art
The modern papermaker employs a highly sophisticated machine to make paper, which is named rather appropriately a "papermaking machine". The modern papermaking machine is in essence a device for removing water from the paper furnish. The water is removed sequentially in three stages or sections of the machine. In the first or forming section, the furnish is deposited on a moving forming wire and water drained through the wire to leave a paper sheet or web having a solids content of circa 18 to 25 percent by weight. The formed web is carried into a wet press felt section and passed through one or more nip presses on a moving press felt to remove sufficient water to form a sheet having a solids content of 36 to 44 percent by weight. This sheet is then transferred to the dryer section of the papermaking machine where dryer felts press the paper sheet against hot, steam-heated dryer cylinders to obtain about 92 to 96 percent solids content.
The clothing employed on the papermaking machine must perform a widely diverse range of functions, according to the position on the machine, i.e., forming, press or dryer section. In view of the diversity of functions, the clothing for use in each section of the machine must be manufactured to meet specific design requirements essential to the particular section. In the absence of meeting the specific felt design requirements demanded in each section of the machine, the overall operation of the machine will be unsatisfactory. Optimum operating lives of the felts will not be achieved, product quality may be adversely affected, machine speeds may be lowered or drying efficiency may be impeded.
Those skilled in the art have long appreciated that the efficiency of water removal in the wet press section of the papermaking machine is critical to overall efficiency in the papermaking process. This is because, first a large amount of water must be removed from the sheet at the presses to realize a good drying economy. Secondly, greater efficiency in water removal creates a drier and hence stronger sheet less susceptible to breaking. A large variety of clothing constructions have been proposed as papermakers felts advantageously employed in the press section of a paper-making machine. In fact, there has been a continual evolution of clothing constructions, corresponding to improvements in the papermaking machine itself. This evolution began with the early woven felt, woven of spun yarn and then mechanically felted or fulled. A later development was found in the "Batt-on-Base" construction consisting of a woven fabric base and a batt surface attached by needling. The needled batt-on-base felts are widely used today and have been said to be the "standard of the industry". However, a wide variety of other constructions are available, including non-woven press felts.
Important physical properties of a papermakers press felt are measured by four test measurements. They are:
1. Saturated moisture: a measure of the amount of water absorbed by the felt under static conditions. Expressed as pounds of water absorbed per pound of felt, saturated moisture is an excellent indicator of the ability of a felt to receive water from the sheet in the nip.
2. Vacuum dewatering: measures the ability of a felt or fabric running on a press to release water to a suction pipe.
3. Air permeability: measured in a dry felt, is expressed as cfm/sq. ft. of felt at 0.5 in. water pressure (m.sup.3 /m.sup.2 per hr. at 10 mm water gauge).
4. Flow resistance: the water permeability of the felt or fabric.
Generally, the batt-on-base felts are advantageous in all four parameters, compared to the earlier conventional woven felt. However, as the speed of the papermaking machines has increased, so has the need for press felts which show an advantage in one or more of the desired physical properties.
One type of press felt which has been suggested is a composite of a woven or non-woven fabric base bearing a surface layer of a flexible, open-cell, polymeric resin foam. This layer, acting like a sponge would enhance the removal of water from the paper sheet. In addition, the inherent thermal insulation provided by the foam layer would impart some protection to the underlying fabric structure which is normally exposed completely to the degradative, hot water being pressed from the paper sheet. These composite felts have also shown good resistance to compaction. Representative of the prior art concerned with the latter composite papermakers felts are the disclosures found in U.S. Pat. Nos. 1,536,533; 2,038,712; 3,059,312; 3,399,111; and 3,617,442. In general, the papermakers felts of the prior art which comprise a composite laminate of a textile and a polymeric resin layer have not been completely satisfactory in regard to their resistance to wear, delamination and long term resistance to compaction. Apparently, the diverse nature of the two components enhances degradation of the overall composite. Further, the presence of a seam in the foam layer serves to provide a weak point in the construction.
The composite structure of the papermakers felts of the present invention are an improvement over many of the prior art composite felts in regard to their resistance to wear, delamination and long term compaction resistance. They are virtually seam free. In addition, the method of their manufacture is an improvement over prior art manufacturing processes for composite felts.